I'm going to assume you are using SSRS here....if so you have to be careful when passing parameters from one report to the other using mdx (for a number of reasons including how the sub report expects to consume the parameter) - try changing the expression of the value you are passing to be fields!whateveryouarepassing.UniqueName as a starter for ten.

If you are passing through the value of a parameter already selected and it is multi-value, SSRS will default to the first item in the array (0). Remove this to pass them all.